It is well known that oil wells have a limited oil producing life and that beginning from the first oil production, a steady progression of declining production is characteristic. Normally, but not necessarily, an oil well will begin its oil production life producing clean oil but eventually water in increasing amounts will also be produced until the water percentage is so great as to cause continued production to be infeasible. Decline of formation pressure caused by fluid withdrawals may also result in less fluid entering the well bore.
In those cases, particularly in oil reservoirs having underlying water, where the well produces increasing amounts of water and decreasing amounts of oil, water coning is occurring. Water breaks into the well bore and as it does, pushes the oil back from the well bore effectively preventing further oil production.
The original water-oil contact is held in balance by upward gradients in the oil zone which is the energy that will drive the oil into the well bore. These gradients are increased when the pressure is reduced inside the well bore by pumping and a water cone beings to rise above the original water-oil contact. The growth of a water cone is schematically depicted in FIG. 1. Ultimately, the tip of the cone encounters increasingly steep pressure gradients so as to overcome the downward differential-gravity gradient between the oil and water causing the cone to break into the well bore.
It is the object of the present invention to intercede the top portion of the water cone with the top of the sump so that the water can be drawn off and out of the well bore through the sump and to prevent the water cone from rising above the top of the sump. This in turn will prevent the water cone from pushing oil back from the well bore while at the same time leaving the formation face above the top of the sump free of water which will allow oil to enter the well bore unimpeded. Consequently both oil and water will enter the sump and be pumped together to the surface, the oil to be saved and marketed, the water to be reinjected or used in another manner.
Gravity drainage is applicable to oil reservoirs and is known to be a very efficient and effective driving mechanism.
Since it is a purpose of the sump to constantly maintain a fluid level in the well bore below the top of the formation, that part of the formation between its top and the well bore fluid level, this distance being the measure of the driving head, is draining fluid out of the formation by the force of gravity which is in addition to any other forces acting to move formation fluids into the well bore. Accordingly, it is an objective of this invention to prevent any inference with the energy of gravity or any other energy driving fluid into the well bore. This is illustrated by FIG. 2 which schematically represents an oil bearing formation with underlying water, showing the measure of the driving head, he-hw, "he" meaning thickness of the formation and "hw" meaning water height; the area A where fluids enter the well bore the zone drained through the sump of movable fluids represented by the triangle x, y, z.
It is an object of this invention to shield the pump from having direct straight line contact with fluids having high water content. The walls of the sump which enclose the pump act as a shield since the fluids must rise in the bore hole high enough to flow over the lip of the sump and down to the pump. Without the sump to act as a shield, fluids will be drawn predominately from that portion of the formation yielding predominately water (from either underlying water or from the water cone) and the well will yield all, or predominately water at the expense of oil production. Also, since the sump is physically positioned in the well bore to be adjacent that part of the formation yielding predominately water, the walls of the sump will tend to impede the flow of water by back pressure created and by the creation of a damming effect, both result tending to reduce the volume of water that would otherwise enter the bore hole. At the same time, oil will move more freely into the well bore from that portion of the formation above the top of the sump. This is illustrated by FIG. 3, schematically depicts water entering the well bore to be drawn off at B, section A-B indicatiang where oil enters the well bore, unimpeded by water, and section B-C showing where the back pressure and damming effect will occur.
It is an object of the present invention to provide a more effective well production system for use with secondary oil recovery in those cases where the oil formation exhibits high porousness and permeability in conjunction with the use of high water injection pressures. In these situations a condition is created where the oil bank may become established and move so rapidly that the producing wells may exhibit a brief increase of oil production and shortly thereafter turn to all water production. Conventional pumping systems using walking beam pump jack, sucker rods and downhole insert type pumps have a narrow range of operating conditions compared to pumping systems using a submersible pump with the automatic on-off switching system. The submersible pump with sump and automatic on-off switching system of this invention has a much broader range of fluid producing capacity and will substantially prevent the result which occurs when a secondary oil bank overrides and by-passes the conventional walking beam pump jack, sucker rod, downhole insert type pump system. Another objective of the invention is to prevent, after an oil bank is present beneath the producing wells, the possibility is that water cones may form rapidly enough to result in a producing well prematurely going to water production.
It is an objective of the present invention to provide a more effective oil production system in those cases where the oil formation exhibits high permeability and wherein the horizontal permeability and the vertical permeability are very nearly of the same values. This condition, which results in very rapid water coning in even early production, is characterized with very high amounts of water with an attendant low amount of oil being pumped. Use of the combination of this invention of the oil well sump enclosing the submersible pump positioned between the top of the formation and the original oil-water contact results in an increase of oil production since the sump causes most of the water to be drawn from the formation at a point approximately even with or a little above the top of the sump while at the same time reducing any impediment to the flow of oil into the well bore from above the top of the sump.
It is another objective of the present invention to provide a more effective production system in those cases where the bore hole has been inadvertently drilled crooked. The conventional pumping system with walking beam pump jack, sucker rod, downhole insert type pump consists of a pumping action whereby the sucker rod string moves up and down inside the production tubing. In a crooked hole both the production tubing and the sucker rod string bend together as they must to conform to the crookedness of the hole. At the points of bends the sucker rod string rubs against the tubing with an up and down motion which eventually abraids a hole in the production tubing or abraids through the sucker rod string requiring ever re-occurring repair work. The sump submersible pump combination, not having up-down moving parts, can be lowered in crooked holes that will not tolerate a production tubing-sucker rod string system.
It is an objective of the present invention to provide a fully automatic means to keep the bore hole voided at all times above the sump through the use of switches actuated on-off by responding to the position of the fluid level in the sump.